Melt 2 tablespoons butter in a large skillet; add mushrooms, and sauté until tender; drain well. Return mushrooms to skillet; add chicken and next 3 ingredients.
Melt 1/3 cup butter in a heavy saucepan over low heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute. Gradually whisk in broth and whipping cream; cook over medium heat, whisking constantly, until thickened and bubbly. Stir in salt and pepper. Add to chicken mixture, stirring gently. Spoon half of chicken mixture into a lightly greased 13- x 9-inch baking dish. Top with oysters and remaining chicken mixture.
Stir together 2 tablespoons melted butter and breadcrumbs; sprinkle over casserole.
Bake at 350° for 30 minutes or until bubbly.